1. Field of the Invention
The present invention is directed to a computer tomography apparatus, and in particular to a computer tomography apparatus having an annularly guided electron beam.
2. Description of the Prior Art
Computer tomography systems are known having an annular x-ray source surrounding a measuring field. The x-ray source has an annular electrode, which is scanned by an electron beam for generating a rotating x-ray beam. The electron gun for injecting the electron beam is disposed on the central axis of the measuring field, which is coincident with the central axis of the annular electrode. The electron gun is disposed on the central axis at distance along the axis from the annular anode, so that the x-ray source has a funnel-shape, and thus has a relatively large structural length.
As an alternative to this known structure, it is possible to inject the electron beam tangentially into the x-ray source, with the electron gun being disposed in the plane of the annular anode, or immediately next to the plane. The electron beam is annularly guided by deflection means, such as deflecting coils, so as to be deflected onto the annular anode for scanning thereof. The electron beam is incident on the anode at a focal spot, and causes the generation of an x-ray beam emanating from that focal spot on the annular anode. The x-ray beam is gated so as to be fan-shaped, and, by the deflection of the electron beam the focal spot, and thus the x-ray beam, is caused to rotates around the central axis of the measuring field so as to scan an examination subject in the measuring field from different angles. An annular x-ray detector acquires the x-rays attenuated by the examination subject, with the intensity of the incident radiation being converted into corresponding electrical signals, which are supplied to a computer which calculates an image of the examined slice of the examination subject therefrom.
The electron beam will typically have a current intensity on the order of magnitude of up to 1 A, given energies of 100 through 120 KeV. In order to obtain the focal spot necessary for computer tomography, the cross section of the electron beam can be circular or elliptical, with a diameter in the range of approximately 0.5 through 1 mm. In computer tomography, it is required that the x-ray beam rotate around the patient with a scan time of 30 through 100 ms, during which time a minimum angle of 180.degree., plus the beam fan angle, for example a total of 220.degree., must be described.
In order to guide the electron beam annularly, deflection means such as coils must be provided. As a consequence of space charge, however, the electron beam will nonetheless tend to widen, and such widening cannot be completely compensated by the deflection means functioning as a focusing device.
The space charge of the electrons can be compensated or neutralized by ions. For example, this can be accomplished by introducing gases into the vacuum vessel of the x-ray source, at pressures on the order of magnitude of 10.sup.-5 through 10.sup.-6 millibars. The gases are ionized by interaction with the electron beam. So-called residual gas ions which arise in this manner are generated in the time of few microseconds after the electron beam is activated, and thereafter cause a self-focusing of the electron beam.
If the electron beam were to employed in the manner of a trace beam for the x-ray generation, such ionization would not build up, and the necessary self-focusing would not occur. The electron beam would diverge, and would impinge the walls of the housing after a few centimeters.
The focusing demands are only slightly less critical if the electron beam is guided in an annular channel having a diameter larger than 1 mm, i.e., for example, a few millimeters. With suitable, additional magnetic fields, an adaptation of the focusing must then be undertaken when the electrons are injected into their path of travel, and when they are deflected onto the annular electrode. In this case, however, it is difficult to generate a focal spot size which satisfies the demands of computer tomography, and which is constant over the entire annular anode.